Low-molecular-ratio cryolite for aluminium electrolytic industry and method for preparing the same

ABSTRACT

The disclosure provides low-molecular-ratio cryolite for aluminium electrolytic industry, which consists of potassium cryolite and sodium cryolite with a mole ratio of 1:1˜1:3, wherein the molecular formula of the potassium cryolite is mKF.AlF 3  and the molecular formula of the sodium cryolite is nNaF.AlF 3 , where m=1˜1.5 and n=1˜1.5. When the low-molecular-ratio cryolite provided by the disclosure is applied to the aluminium electrolytic industry, electrolytic temperature and power consumption can be reduced and electrolytic efficiency is improved.

TECHNICAL FIELD OF THE INVENTION

The disclosure relates to cryolite, and in particular to the low-molecular-ratio cryolite for aluminium electrolytic industry and a method for preparing the same.

BACKGROUND OF THE INVENTION

At present, aluminium electrolytic industry still employs a conventional Hall-Heroult process; electrolyte always takes cryolite-aluminium oxide as a basic system, and the cryolite generally adopts sodium fluoroaluminate. The aluminium electrolytic industry needs an electrolytic temperature of about 960 DEG C and thus power consumption is high, this is mainly because the liquidus temperature of the electrolyte is high and it is necessary to keep a certain temperature of superheat degree to make the aluminium oxide have a better solubility.

The method for preparing cryolite in industry generally adopts a synthesis method, in which anhydrous hydrofluoric acid reacts with aluminium hydroxide to form fluoaluminic acid; then the fluoaluminic acid reacts with sodium hydroxide or potassium hydroxide at a high temperature; after processes of filtering, drying, melting and crushing, the cryolite is prepared, wherein the cryolite synthesized by this method has a molecular ratio of m=3.0, with a relatively high melting point. The cryolite synthesized by the existing industrial synthesis method has a molecular ratio of m=2.0-3.0, and it is difficult to obtain the relatively pure low-molecular-ratio cryolite containing extremely low water content with a molecular ratio of m=1.0-1.5.

Therefore, the conventional art has disadvantages that the electrolytic power consumption is high and the electrolyte is not ideal.

SUMMARY OF THE INVENTION

In order to solve the technical problem existing in the conventional art, the inventor has done a great deal of research in the selection and preparation of electrolyte and unexpectedly finds that taking the mixture of low-molecular-ratio potassium cryolite and low-molecular-ratio sodium cryolite with a certain ratio as the electrolyte of the aluminium electrolytic system can significantly reduce the electrolytic temperature compared with the conventional aluminium electrolytic system which takes sodium cryolite as the electrolyte, and has obvious advantages in corrosion to electrode materials compared with the aluminium electrolytic system which takes single low-molecular-ratio potassium cryolite or low-molecular-ratio sodium cryolite as the electrolyte, but has the electrolytic temperature decrease fallen in between the conventional aluminium electrolytic system which takes sodium cryolite as the electrolyte and the aluminium electrolytic system which takes single low-molecular-ratio potassium cryolite or low-molecular-ratio sodium cryolite as the electrolyte.

The disclosure provides low-molecular-ratio cryolite for aluminium electrolytic industry, which consists of potassium cryolite and sodium cryolite with a mole ratio of 1:1˜1:3, wherein the molecular formula of the potassium cryolite is mKF.AlF₃, m=1˜1.5; the molecular formula of the sodium cryolite is nNaF.AlF₃, n=1˜1.5.

With the technical scheme above, when the low-molecular-ratio cryolite provided by the disclosure is applied to the aluminium electrolytic industry, the solubility property of aluminium oxide is improved, thus, the electrolytic temperature is reduced, the power consumption is reduced and the electrolytic efficiency is improved.

As a further improvement of the disclosure, m=1, 1.2 or 1.5; when m=1.0˜1.5; the melting point of the potassium cryolite mKF.AlF₃ is between 540 and 570 DEG C, wherein the melting point of the mKF.AlF₃ slightly increases as the increase of m. n=1, 1.2 or 1.5; when n=1.0˜1.5, the melting point of the nNaF.AlF₃ is between 960 and 1000 DEG C, wherein the melting point of the sodium cryolite nNaF.AlF₃ slightly increases as the increase of n.

As a further improvement of the disclosure, the mole ratio of the potassium cryolite to the sodium cryolite is 1:1; m=1.5 and n=1.5; the aluminium oxide has a solubility of 13 g/l in the system consisting of potassium cryolite and sodium cryolite with a mole ratio of 1:1 and the electrolytic temperature is between 825 and 900 DEG C.

Correspondingly, the disclosure also provides a method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry, which includes the following steps:

A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding potassium fluotitanate, potassium fluoborate or mixture of they two in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite; putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding sodium fluotitanate, sodium fluoborate or mixture of they two in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite; and

B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.

The preparation method provided by the disclosure has advantages of mild reaction conditions, easy control, simple process, full reaction and high quality of reaction product.

As a further improvement of the disclosure, the method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry includes the following steps:

A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 780 and 850 DEG C, adding potassium fluotitanate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is

${\frac{3}{2}{{KF} \cdot {AlF}_{3}}};$

putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 780 and 850 DEG C, adding sodium fluotitanate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is

${\frac{3}{2}{{NaF} \cdot {AlF}_{3}}};$

and

B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3, wherein the reaction formula involved is:

${{{\frac{3}{4}K_{2}{TiF}_{6}} + {Al}} = {{\frac{3}{4}{Ti}} + {\frac{3}{2}{{KF} \cdot {AlF}_{3}}}}};$ ${{\frac{3}{4}{Na}_{2}{TiF}_{6}} + {Al}} = {{\frac{3}{4}{Ti}} + {\frac{3}{2}{{NaF} \cdot {{AlF}_{3}.}}}}$

As a further improvement of the disclosure, the method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry includes the following steps:

A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding potassium fluoborate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is KF.AlF₃; putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding sodium fluoborate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is NaF.AlF₃; and

B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3, wherein the reaction formula involved is:

KBF₄+Al=B+KF.AlF₃; NaBF₄+Al=B+NaF.AlF₃.

As a further improvement of the disclosure, the method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry includes the following steps:

A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of potassium fluoborate and potassium fluotitanate with a mole ratio of 2:1 in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is

${\frac{6}{5}{{KF} \cdot {AlF}_{3}}};$

putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of 2:1 in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is

${\frac{6}{5}{{NaF} \cdot {AlF}_{3}}};$

and

B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3, wherein the reaction formula involved is:

${{{K_{2}{TiF}_{6}} + {2{KBF}_{4}} + {\frac{10}{3}{Al}}} = {{TiB}_{2} + {\frac{10}{3}\left\lbrack {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right\rbrack}}};$ ${{{Na}_{2}{TiF}_{6}} + {2{NaBF}_{4}} + {\frac{10}{3}{Al}}} = {{TiB}_{2} + {{\frac{10}{3}\left\lbrack {\frac{6}{5}{{NaF} \cdot {AlF}_{3}}} \right\rbrack}.}}$

As a further improvement of the disclosure, the method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry includes the following steps:

A) putting excessive aluminium into a reactor, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of potassium fluoborate and potassium fluotitanate with a mole ratio of y:x in the reactor and stirring for 0.5 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is

${\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {AlF}_{3}}};$

putting excessive aluminium into another reactor, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of y:x in the reactor and stirring for 0.5 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is

${\frac{{3y} + {6x}}{{3y} + {4x}}{{NaF} \cdot {AlF}_{3}}};$

and

B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3, wherein the reaction formula involved is:

${{K_{2}{TiF}_{6}} + {KBF}_{4} + {{{Al}{Al}} \cdot {Ti} \cdot B} + {\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {AlF}_{3}}}};$ ${{Na}_{2}{TiF}_{6}} + {NaBF}_{4} + {{{Al}{Al}} \cdot {Ti} \cdot B} + {\frac{{3y} + {6x}}{{3y} + {4x}}{{NaF} \cdot {{AlF}_{3}.}}}$

Compared with the conventional art, the disclosure achieves advantages as follows: when the low-molecular-ratio cryolite provided by the disclosure is applied to the aluminium electrolytic industry, the solubility property of aluminium oxide is improved, thus, the electrolytic temperature is reduced; and compared with the aluminium electrolytic system which takes the conventional cryolite or single low-molecular-ratio potassium cryolite or single low-molecular-ratio sodium cryolite as the electrolyte, the electrolytic temperature is obviously different and the corrosion to electrode materials is different too; the method provided by the disclosure for preparing the low-molecular-ratio cryolite has advantages of mild reaction conditions, easy control, simple process and full reaction.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosure is described below in further detail through specific embodiments.

Embodiment 1

Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 800 DEG C, adding dried potassium fluotitanate in the reactor slowly in accordance with a reaction ratio and stirring quickly for 5 hours to form titanium sponge and potassium cryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right),$

opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite through a siphon-pump. Weighing 1 ton of aluminium and putting it into another reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 800 DEG C, adding dried sodium fluotitanate in the reactor slowly in accordance with a reaction ratio and stirring quickly for 5 hours to form titanium sponge and sodium cryolite

$\left( {\frac{3}{2}{{NaF} \cdot {AlF}_{3}}} \right),$

opening the cover of the reactor, pumping the superstratum melt liquid sodium cryolite through a siphon-pump.

Mixing the prepared potassium cryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right),$

with the prepared sodium cryolite

$\left( {\frac{3}{2}{{NaF} \cdot {AlF}_{3}}} \right),$

in a mole ratio of 1:1 and applying the cryolite mixture to the aluminium electrolytic industry, wherein the electrolytic temperature can be controlled in a range of between 850 and 900 DEG C, and virgin aluminium can be obtained by using inert electrode materials or carbon electrode materials or mixed (combination of carbon and inert electrode materials) electrode materials to carry out electrolysis.

Embodiment 2

Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 780 DEG C, adding dried potassium fluoborate in the reactor slowly in accordance with a reaction ratio and stirring quickly for 5 hours to form boron and potassium cryolite (KF.AlF₃), opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite through a siphon-pump. Weighing 1 ton of aluminium and putting it into another reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 780 DEG C, adding dried sodium fluoborate in the reactor slowly in accordance with a reaction ratio and stirring quickly for 5 hours to form boron and sodium cryolite (NaF.AlF₃), opening the cover of the reactor, pumping the superstratum melt liquid sodium cryolite through a siphon-pump.

Mixing the prepared potassium cryolite (KF.AlF₃) with the prepared sodium cryolite (NaF.AlF₃) in a mole ratio of 1:1 and applying the cryolite mixture to the aluminium electrolytic industry, wherein the electrolytic temperature can be controlled in a range of between 825 and 900 DEG C, and virgin aluminium can be obtained by using inert electrode materials or carbon electrode materials or mixed (combination of carbon and inert electrode materials) electrode materials to carry out electrolysis.

Embodiment 3

Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 750 DEG C, adding the mixture of dried potassium fluoborate and potassium fluotitanate in the reactor slowly in accordance with a reaction ratio, wherein the mole ratio of the potassium fluoborate to the potassium fluotitanate is 2:1; stirring quickly for 5 hours to form titanium boride and potassium cryolite

$\left( {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right),$

opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite through a siphon-pump. Weighing 1 ton of aluminium and putting it into a reactor, injecting argon to the reactor for protection after vacuumizing, heating the reactor to a temperature of 750 DEG C, adding the mixture of dried sodium fluoborate and sodium fluotitanate in the reactor slowly in accordance with a reaction ratio, wherein the mole ratio of the sodium fluoborate to the sodium fluotitanate is 2:1; stirring quickly for 5 hours to form titanium boride and sodium cryolite

$\left( {\frac{6}{4}{{NaF} \cdot {AlF}_{3}}} \right),$

opening the cover of the reactor, pumping the superstratum melt liquid sodium cryolite through a siphon-pump.

Mixing the prepared potassium cryolite

$\left( {\frac{6}{5}{{NaF} \cdot {AlF}_{3}}} \right)$

with the prepared sodium cryolite

$\left( {\frac{6}{5}{{NaF} \cdot {AlF}_{3}}} \right)$

in a mole ratio of 1:1 and applying the cryolite mixture to the aluminium electrolytic industry, wherein the electrolytic temperature can be controlled in a range of between 825 and 900 DEG C, and virgin aluminium can be obtained by using inert electrode materials or carbon electrode materials or mixed (combination of carbon and inert electrode materials) electrode materials to carry out electrolysis.

Embodiment 4

Mixing the prepared potassium cryolite (KF.AlF₃) with the prepared sodium cryolite

$\left( {\frac{6}{5}{{NaF} \cdot {AlF}_{3}}} \right)$

in a mole ratio of 1:3 and applying the cryolite mixture to the aluminium electrolytic industry, wherein the electrolytic temperature can be controlled in a range of between 850 and 900 DEG C, and virgin aluminium can be obtained by using inert electrode materials or carbon electrode materials or mixed (combination of carbon and inert electrode materials) electrode materials to carry out electrolysis.

Embodiment 5

Mixing the prepared potassium cryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right)$

with the prepared sodium cryolite (NaF.AlF₃) in a mole ratio of 1:3 and applying the cryolite mixture to the aluminium electrolytic industry, wherein the electrolytic temperature can be controlled in a range of between 850 and 900 DEG C, and virgin aluminium can be obtained by using inert electrode materials or carbon electrode materials or mixed (combination of carbon and inert electrode materials) electrode materials to carry out electrolysis.

Embodiment 6

Mixing the prepared potassium cryolite

$\left( {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right)$

with the prepared sodium cryolite

$\left( {\frac{3}{2}{{NaF} \cdot {AlF}_{3}}} \right)$

in a mole ratio of 1:3 and applying the cryolite mixture to the aluminium electrolytic industry, wherein the electrolytic temperature can be controlled in a range of between 850 and 900 DEG C, and virgin aluminium can be obtained by using inert electrode materials or carbon electrode materials or mixed (combination of carbon and inert electrode materials) electrode materials to carry out electrolysis.

Embodiment 7

Weighing 5 tons of aluminium and putting it into a reactor, heating the reactor to a temperature of 750 DEG C, adding 2 tons of mixture of dried potassium fluoborate and potassium fluotitanate in the reactor slowly, wherein the mole ratio of the potassium fluoborate to the potassium fluotitanate is 1:1; stirring quickly for 4 hours to form aluminium-titanium-boron alloy and potassium cryolite

$\left( {\frac{9}{7}{{KF} \cdot {AlF}_{3}}} \right)$

due to excessive aluminium, opening the cover of the reactor, pumping the superstratum melt liquid potassium cryolite through a siphon-pump. Weighing 5 tons of aluminium and putting it into a reactor, heating the reactor to a temperature of 750 DEG C, adding 2 tons of mixture of dried sodium fluoborate and sodium fluotitanate in the reactor slowly, wherein the mole ratio of the sodium fluoborate to the sodium fluotitanate is 1:1; stirring quickly for 4 hours to form aluminium-titanium-boron alloy and sodium cryolite

$\left( {\frac{9}{7}{{NaF} \cdot {AlF}_{3}}} \right)$

due to excessive aluminium, opening the cover of the reactor, pumping the superstratum melt liquid sodium cryolite through a siphon-pump.

Mixing the prepared potassium cryolite

$\left( {\frac{9}{7}{{KF} \cdot {AlF}_{3}}} \right)$

with the prepared sodium cryolite

$\left( {\frac{9}{7}{{NaF} \cdot {AlF}_{3}}} \right)$

in a mole ratio of 1:3 and applying the cryolite mixture to the aluminium electrolytic industry, wherein the electrolytic temperature can be controlled in a range of between 850 and 900 DEG C, and virgin aluminium can be obtained by using inert electrode materials or carbon electrode materials or mixed (combination of carbon and inert electrode materials) electrode materials to carry out electrolysis.

The above are the further detailed description of the disclosure made in conjunction with specific preferred embodiments; it can not be considered that the specific embodiment of the disclosure is only limited to the description above. For the common technicians in the technical field of the disclosure, empty simple deductions or substitutes can be made without departing from the concept of the disclosure and they are deemed to be included within the scope of protection of the disclosure. 

What is claimed is:
 1. Low-molecular-ratio cryolite for aluminium electrolytic industry, which consists of potassium cryolite and sodium cryolite with a mole ratio of 1:1˜1:3, wherein the molecular formula of the potassium cryolite is mKF.AlF₃, m=1˜1.5; the molecular formula of the sodium cryolite is nNaF.AlF₃, n=1˜1.5.
 2. The low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 1, wherein m=1, 1.2 or 1.5; n=1, 1.2 or 1.5.
 3. The low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 1, wherein the mole ratio of the potassium cryolite to the sodium cryolite is 1:1; m=1.5 and n=1.5.
 4. A method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 1, including the following steps: A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding potassium fluotitanate, potassium fluoborate or mixture of they two in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite; putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding sodium fluotitanate, sodium fluoborate or mixture of they two in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite; and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 5. The method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 4, including the following steps: A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 780 and 850 DEG C, adding potassium fluotitanate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is $\frac{3}{2}{{KF} \cdot {AlF}_{3}}$ putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 780 and 850 DEG C, adding sodium fluotitanate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is ${\frac{3}{2}{{NaF} \cdot {AlF}_{3}}};$ and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 6. The method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 4, including the following steps: A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding potassium fluoborate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is KF.AlF₃; putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding sodium fluoborate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is NaF.AlF₃; and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 7. The method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 4, including the following steps: A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of potassium fluoborate and potassium fluotitanate with a mole ratio of 2:1 in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is ${\frac{6}{5}{{KF} \cdot {AlF}_{3}}};$ putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of 2:1 in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is ${\frac{6}{5}{{NaF} \cdot {AlF}_{3}}};$ and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 8. The method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 4, including the following steps: A) putting excessive aluminium into a reactor, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of potassium fluoborate and potassium fluotitanate with a mole ratio of y:x in the reactor and stirring for 0.5 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is ${\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {AlF}_{3}}};$ putting excessive aluminium into another reactor, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of y:x in the reactor and stirring for 0.5 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is ${\frac{{3y} + {6x}}{{3y} + {4x}}{{NaF} \cdot {AlF}_{3}}};$ and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 9. A method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 2, including the following steps: A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding potassium fluotitanate, potassium fluoborate or mixture of they two in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite; putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding sodium fluotitanate, sodium fluoborate or mixture of they two in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite; and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 10. The method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 9, including the following steps: A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 780 and 850 DEG C, adding potassium fluotitanate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is ${\frac{3}{2}{{KF} \cdot {AlF}_{3}}};$ putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 780 and 850 DEG C, adding sodium fluotitanate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is ${\frac{3}{2}{{NaF} \cdot {AlF}_{3}}};$ and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 11. The method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 9, including the following steps: A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding potassium fluoborate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is KF.AlF₃; putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding sodium fluoborate in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is NaF.AlF₃; and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 12. The method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 9, including the following steps: A) putting aluminium into a reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of potassium fluoborate and potassium fluotitanate with a mole ratio of 2:1 in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is ${\frac{6}{5}{{KF} \cdot {AlF}_{3}}};$ putting aluminium into another reactor, injecting an inert gas to the reactor after vacuumizing, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of 2:1 in the reactor and stirring for 4 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is ${\frac{6}{5}{{NaF} \cdot {AlF}_{3}}};$ and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3.
 13. The method for preparing the low-molecular-ratio cryolite for aluminium electrolytic industry according to claim 9, including the following steps: A) putting excessive aluminium into a reactor, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of potassium fluoborate and potassium fluotitanate with a mole ratio of y:x in the reactor and stirring for 0.5 to 6 hours, pumping the superstratum melt liquid to obtain potassium cryolite of which the molecular formula is ${\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {AlF}_{3}}};$ putting excessive aluminium into another reactor, heating the reactor to a temperature of between 700 and 850 DEG C, adding the mixture of sodium fluoborate and sodium fluotitanate with a mole ratio of y:x in the reactor and stirring for 0.5 to 6 hours, pumping the superstratum melt liquid to obtain sodium cryolite of which the molecular formula is ${\frac{{3y} + {6x}}{{3y} + {4x}}{{NaF} \cdot {AlF}_{3}}};$ and B) mixing the obtained potassium cryolite with the obtained sodium cryolite in a mole ratio of 1:1˜1:3. 